If you don't eat the entire pizza and want to keep the rest of the pizza for the next day you really should refrigerate it and the pizza will keep for 3 to 4 days refrigerated although it begins to get stale the pizza will still be safe to eat.
Although I have eaten some pizza that had cheese and meats on it that I had left out overnight when I was a teenager and not thinking correctly nor was I worried about getting sick from the food I was eating.
But after eating the pizza that I left out overnight nothing happened to me and I didn't get sick or even get a stomach ache.
So if the pizza was just left out overnight for one night then it's unlikely it would make you sick but if you're in doubt then just throw it out.
But I haven't gotten sick eating leftover pizza that had been left out overnight.
I've done it in my adult years accidentally as well and the pizza was still good to eat.